[quote=thewizardguy]
Remember, it's possible to make your soul indestructible, like with the Daemon or Lord Cucumber, but they should always have a weakness.
[/quote]

And in this case, Thyela's soul is substantially weaker that Kaika's soul in the terms of indestructive souls, since her soul is corrupted by the essence of Shadow replicas. Since those two essences (her pact and her Shadow Replica essence) literally dont mix together that well, Thyela has pretty much an unstable soul that can be manipulated easily.